The objective of this research is to obtain an understanding of the role of lipids, especially the phospholipid polar head groups, in membrane structure and function. The main approach to be used in this research will be to alter the lipid composition with respect to the fatty acids, the polar head groups and the sterol content of cells in tissue culture. Subcellular membrane fractions, including the plasma membrane, endoplasmic reticulum, and mitochondria will be isolated from the cells with different lipid compositions. The changes in the lipid composition will be correlated with changes in the physical and biochemical properties of the membranes. The physical properties of the membranes will be studied by a number of spectroscopic methods: Fluorescence, circular dichroism, and deuterium magnetic resonance. Two lipid-requiring enzyme system, adenylate cyclase and the microsomal electron transport system, will be studied to determine the effect of altering the lipid composition on their activity. In addition, studies will be directed towards understanding the mechanism by which enveloped viruses, particularly Rous sarcoma virus, bud from the cells. Attention will be focused on the existence and origin of localized lipid regions in the plasma membrane where viral budding occurs. These studies should provide unique information on the contribution of lipids to the activity of membranous enzymes and the significance of the lipid composition for the fluidity and structure of animal cell membranes and viruses.